High frequency inductance coil assembly



Nov. 1, 1955 H. M. VISCH HIGH FREQUENCY INDUCTANCE con ASSEMBLY Filed Nov. 15, 1949 United States Patent HIGH FREQUENCY INDUCTANCE COIL ASSEMiiLY Henri Marinus Visch, Eindhoven, Netherlands, assignor to Hartford National Bank 62 Trust Company, Hartford, Conn., as trustee Application November 15, 1949, Serial No. 127,358

Claims priority, application Netherlands November 20, 1948 1 Claim. (Cl. 336-87) The invention relates to a high-frequency inductance coil assembly provided with a magnetic shield.

As is well-known, magnetic shields are provided around high-frequency inductance coils to screen such coils from the action of stray fields and to reduce the leakage field of the coil. For this purpose, tubular shields consisting of high-frequency iron have been widely employed to provide a magnetic shield with virtually no losses. Such shields, moreover, increase the inductance of the coil. These shields have the disadvantage, however, that for desired mechanical rigidity, the walls of the shield must be made relatively thick, thus increasing the quantity of material required and, incidentally, thus increasing the cost of the coil.

According to the invention, a high-frequency coil assembly is provided with a magnetic shield assembly which comprises a plurality of rod-shaped elements consisting of a sintered ceramic ferromagnetic material having a magnetic permeability exceeding 100, which are uniformly positioned spaced apart around the coil and extend substantially parallel to the axis of the coil. The sintered ceramic ferromagnetic material is, preferably, a ferrite of the class described in U. S. Patents 2,452,529, 2,452,530 and 2,452,531 issued October 26, 1948, which are known as mixed crystal ferrites and will be referred to hereinafter as a ferrite. These materials are ceramic products obtained by sintering two or more oxides of bivalent metals and ferric oxide at relatively high temperatures as described in those patents, and are characterized by high magnetic permeabilities and low magnetic losses.

Surrounding the coil assembly and the shield is the usual screening envelope, which is generally constituted by a conductive material. Because of the high magnetic permeability of the rod-like elements, the magnetic circuit is substantially closed through them with the result that the leakage fiux is very small. Accordingly, the leakage field which induces eddy-currents in the screening envelope is virtually negligible.

The invention will now be described in connection with the accompanying drawing, in which:

Fig. 1 is an elevational view of a coil assembly according to the invention;

Fig. 2 is a plan view of the coil assembly of Fig. 1 with the casing removed;

Fig. 3 is a plan view of another embodiment of the magnetic shield for the coil assembly according to the invention;

Fig. 4 is a plan view of still another embodiment of the magnetic shield for the coil assembly according to the invention.

Referring to Fig. 1, there is shown an elevational view of a band-pass filter provided with a magnetic shield. The filter is provided with an insulating base 1 for supporting coils 3 and 5 mounted on supporting forms 7 and 9, respectively. Movable within the coils are cores 11 and 13, which consist of ferrite for varying the inductance of the respective coils. Surrounding the coils and uniformly spaced apart are rod-like elements 15, 17, 19, 21, 23, 25 and 27, which consist of a mixed crystal ferrite having a magnetic permeability greater than and serve to shield the coil against leakage fields. Enclosing the entire assembly is a casing 29, preferably of aluminium. The ends of the coils are connected to terminals 31 secured to the underside of the base 1 by rivets 33.

Because of the high magnetic permeability of the ferrite, e. g. about 400, the major portion of the flux issuing from the core ends traverses the rods and completes the magnetic circuit into the opposite ends of the coils. The leakage field remaining is so negligible that the easing 29 can closely surround the coils without the coils inducing appreciable eddy-currents in the casing.

To illustrate the compact size that can be readily realized by means of the foregoing arrangement, dimensions of the components and the relative spacing of the casing 29 and the coils 3 and 5, is indicated in Fig. 2.

The following table illustrates the effect of the magnetic shield. The measurements were on a band-pass filter as described above, at a frequency of about 450 kc./s. with core pins 11 and 13 in both the in position (for maximum inductance), and in the out position (for minimum inductance). The primary and secondary inductances (L) are approximately equal; r represents the H. F.-series resistance.

From the foregoing measurements, it is evident that the product of the coupling factor K and the quality factor of the coil Q, which determines the shape of the transmission curve of the filter, is unaffected by the adjustment of the inductance which, of course, is desirable.

It has been further found that this desirable characteristic results from the presence of the central rod-like element 21. To illustrate that feature, Figs. 3 and 4 show two rod arrangements in which, in one case, the central rod is omitted and in the other case, is provided between the coils. In the arrangement shown in Fig. 3, the rod 21 is provided between the coils and it has been found that the product K Q remains substantially constant, independent of the positioning of the cores. In the arrangement shown in Fig. 4, rod 21 has been omitted and the product K Q has been found to vary between 3.7 and 2.8 as the cores are moved from the in position to the out position. It appears, therefore, that the central rod 21 has a considerable influence on the coupling of the coils. In general, however, it may be stated that by altering the positioning of the rods and their size, a considerable influence on the characteristics of the filter can be effected which, in addition to an economy in the use of the ferrites, is a great advantage.

Moreover, it appears that the arrangement illustrated in Fig. 3 results in approximately the same loss factor r/L as for those shown in Figs. 2 and 4, indicating that rods 17, 19, 23 and 25, which are placed at the vertices of a rectangle and on opposite sides of the coils, have the greatest influence on the quality of the coils. Hence, a considerable saving in material can be effected by employing only those rods together with rod 21 for optimum results.

The invention is particularly useful for band-pass filters, l.-F. transformers and high-frequency inductance units, and results in an appreciable economy of space and materials.

While I have thus described my invention with specific examples and embodiments thereof, other obvious modifications will appear obvious to those skilled in the art without departing from the spirit and scope of the invention.

What I claim is:

A band-pass filter assembly or the like comprising two juxtaposed parallel generally cylindrical coils, cores of ferromagnetic ferrite material movable in the respective coils, a ferromagnetic shield for said coils and spaced from said cores, said shield comprising at least four and not more than six spaced apart rod-shaped elements disposed alongside said coils and extending substantially parallel to the axes thereof, said rod-shaped elements being disposed at spaced intervals to substantially surround said juxtaposed coils, and a further rod-shaped element disposed between the two coils and also extending substantially parallel to the axes of the coils, each of said rod-shaped elements being constituted of a ceramic ferromagnetic ferrite material having a permeability of at least 100, said rod-shaped elements being unconnected by other ferromagnetic material, and a conductive housing closely surrounding the coils, cores and shield.

References Cited in the file of this patent UNITED STATES PATENTS 1,709,054 Bennett Apr. 16, 1929 1,811,466 George et al June 23, 1931 2,225,967 Berman Dec. 24, 1940 2,320,537 Santos June 1, 1943 2,346,584 Jacob Apr. 11, 1944 2,413,201 Tillman Dec. 24, 1946 2,452,530 Snoek Oct. 26, 1948 2,458,282 Maki Jan. 4, 1949 FOREIGN PATENTS 295,502 Great Britain Aug. 16, 1928 402,081 Great Britain May 19, 1933 

